Semiconductor manufacturing often involves processing silicon wafers using multi-step (or stage) manufacturing processes. It is often of great importance for manufacturers to identify and/or track individual wafers as they progress along such manufacturing flows. To facilitate such identification, wafers can be laser marked with serial numbers in the form of bar codes and/or alphanumeric characters, and identified using automated reading systems.
With the recent advent of the semiconductor industry's T7 specification, wafers can be laser marked with two-dimensional (2D) barcodes and redundant character strings (that contain the same information in the 2D barcode). For example, a relatively small 2D data symbol can be laser etched directly under a much larger twelve (12) character string. The character string may physically occupy an area of the wafer that is approximately three (3) to four (4) times the height and width of the 2D data symbol. Given various factors, such as the high value of usable wafer area (and therefore the desire to mark only as little area as necessary for identification), the amount of time required to laser scribe marks, the potential for inducing residual stress in the substrate during marking, and the likely particle generation associated with a laser scribing process, the redundant alphanumeric character string is often viewed as unnecessary and/or wasteful.
Regardless, the redundant character string is often included since prior to the 2D data symbol T7 specification, most wafers were marked with alphanumeric strings alone. Additionally, the character string can be used for error recovery. Because of the reflective nature of silicon substrates and the effect of various process steps on their optical characteristics, semiconductor wafer scribes can be difficult to reliably decode using automated reading systems. Since humans, in general, are not capable of manually decoding 2D barcodes, known methods for error recovery in the identification process rely on the fact that an alphanumeric string is present on the wafer and visible to the operator. For example, should an automated reading system fail to identify a wafer within a predetermined degree of confidence, an operator can assist the automated reading process by manually keying in the characters of the identification string (e.g., using a keyboard). In order to enter the string, the operator usually must be able to personally view, recognize, and key in the character string.
Known methods of wafer read-error recovery can introduce certain inefficiencies in the performance of automated wafer reading systems, which can in turn affect the semiconductor wafer manufacturing process itself. Such inefficiencies can be further compounded by the fact that operators often depend on an image of the character string formed by automated reading equipment (e.g., since it may be difficult or impossible for an operator to view the character string using the unaided human eye). Since the alphanumeric identification string can be several times larger than the optimal field of view required for reading the data symbol alone, the automated reading equipment is often unable to form an image of the alphanumeric identification string in its entirety. While automated wafer reading devices can be optimized to view alphanumeric strings, such modification often comes at the expense of reducing the resolution necessary to read the associated 2D barcode symbols. It is often the 2D barcode symbol, and not the alphanumeric string, that possesses greater potential for error correction and detection (e.g., which can lead to fewer automatic identification faults that require manual intervention).